1. Technical Field
The present invention relates generally to control systems and, more particularly, to a control system for a motor vehicle utilizing proportional/integral/derivative control in conjunction with linearization curves.
2. Discussion
In many control systems, it is desirable to force a process variable to a predetermined set point. For instance, idle speed control and emission control in motor vehicles are examples of feedback control systems which maintain a process variable at a desired set point. According to these processes, an error value is determined by comparing measured values to the desired set point. The measured value is the output of feedback from a system component. Thereafter, a controller is responsive to the error value to control at least one operating parameter of the process to reduce the error value. By controlling the system component according to the measured value, the measured value is changed. Continuing the closed loop control process forces the measured value to the set point.
According to the prior art, proportional/integral/derivative (PID) control theory is utilized in combination with linearization curves. The linearization curve is generated according to theoretical and experimental test data generated for the operating environment in which the control system is to be employed. As illustrated in FIG. 6, prior art linearization curves are generally broken down into three portions. A first steeply sloped portion 100, a second moderately sloped portion 102, and a third steeply sloped portion 104. The target set point 106 is selected along the moderately sloped portion 102 of the linearization curve according to design needs. If the measured value from the application in which the control system is used falls within either the first or third steeply sloped portions 100, 104, the measured value is outside of the acceptable range for the process operating parameters. Therefore, massive corrections to the operating parameters are required to bring the measured value to a location along the second moderately sloped portion 102. These correction techniques are well known in the art and are not the subject of the present application. On the other hand, if the measured value falls along the moderately sloped portion 102 of the linearization curve, less massive corrective measures may be implemented to vary the operating parameters such that the measured value moves to the target set point 106.
According to the prior art, the same corrective measures are implemented for a measured value falling anywhere along the moderately sloped portion 102 of the linearization curve despite its proximity to the desired set point 106. As such, even a measured value which is close to the target set point 106 causes aggressive corrective processing by the control system. This causes overshooting of the target set point 106 which requires further corrections. Overshooting and re-correcting results in large fluctuations of the operating parameters and the measured value oscillates around the target set point 106.
In view of the foregoing, it would be desirable to provide a control system which tailors the magnitude of the corrective measures as the measured value approaches the target set point and implements no corrections when the measured value is within a preselected range of the target set point.